The demographics of atheism and religiosity have some puzzling issues.
The differences between Europe and the US, countries with similar economic development are striking.
https://en.wikipedia.org/wiki/Demographics_of_atheism#Europehttps://en.wikipedia.org/wiki/Demographics_of_atheism#United_StatesThere seems to be a trend on the United States for an increase of non-religious people, but religiosity on the United States is way over the one on Europe.
There are more than one explanation:
1) State religions on Europe reduced competition and allowed some dominant churches to keep defending absurd positions.
2) There was on European eastern countries a state policy against religion under communism (still today, eastern Germany is the most atheist region on the world:
https://en.wikipedia.org/wiki/Demographics_of_atheism#Germany).
But many of European churches' ideas are not more crazy than the American ones. And some of the top atheist countries (like the Scandinavians) never had any state police against religion.
So, the reasons are not obvious.
Anyway, the numbers are not completely favorable to atheism. Basically, atheist have much lower birth rates than religious people.
Some of the reasons for this low birth rate are cultural. Statistics say that atheist have higher levels of education (
http://www.pewresearch.org/fact-tank/2015/11/05/7-facts-about-atheists/). Well, around the world, educated people seem to be actively investing on their own extinction.
Some other are economical. Atheism is predominant on developed countries, where there are (were?) strong social security systems. Having kids is a bad deal, because they are expensive (no more child work) and they aren't needed to assure patrimonial security on old age.
Of course, since social security is going to explode because of these low birth rates and longer life expectation, this will change soon.
Some other reasons are religious: religious people do take seriously the command to multiply themselves.